In an image processing apparatus used for a TV, an image monitor, a digital camera and the like, enhancement processing is performed independently on image data lying in the horizontal direction from a focus pixel and on image data lying in the vertical direction from the focus pixel, on the basis of a change in luminance and/or color of the image data. The enhancement processing is disclosed in Japanese Patent Application Publication No. 2006-304352.
When the enhancement processing is performed on image data lying in a diagonal direction, the enhancement processing has a problem that the image data ends up with a stepwise aliasing line. In a case of noise contamination in image data, a noise in a bright portion of the image data tends to be less unnoticeable whereas a noise in a dark portion tends to be noticeable even if the noises are of the same level. When noise reduction processing is uniformly applied to the whole screen image, there occurs a problem that it is difficult to reduce noises on both a bright portion and a dark portion simultaneously and effectively.